Exoplanet Portraits: Direct Images of Other Worlds

While they may look like no more than fuzzy dots, these are real pictures of other worlds. Over the last few years, astronomers have been learning how to accomplish the difficult task of directly imaging extrasolar planets and have now captured about half a dozen subjects.

Direct imaging is one of the next big vistas in astronomy. Soon, researchers will have telescopes that can focus in greater detail on these exoplanets, providing better information about their atmospheric composition and possibly even detecting the signatures of life.

“The only way we’re going to really nail down an Earth-like planet around a star like our sun is with direct imaging,” said Bruce Macintosh, an astrophysicist at Lawrence Livermore National Laboratory.

Though more than 100 new extrasolar planets have been announced at the Extreme Solar Systems II meeting in Moran, Wyoming this week, all were discovered through indirect methods that detect planets as they dim the light of their parent star as they pass in front of it or gravitationally tug on it to cause a wobble. These techniques can discern a planet’s size and location — important clues to its habitability, but with direct imaging, light from a distant planet can be spread out into a spectrum, providing information about the molecules and compounds in its atmosphere. One day, researchers may find a directly imaged planet that harbors the ingredients of life, such as water and oxygen.

But direct imaging is a tricky business. Scientists need to figure out how to block out the light from a vivid star and resolve the much dimmer light reflected by a planet.

“It’s like trying to see a faint ember glowing next to a very bright searchlight,” said astronomer Ray Jayawardhana of the University of Toronto in Canada, author of the book Strange New Worlds.

To accomplish this task with ground-based telescopes, astronomers use adaptive optics to smooth out the blurring of the distant light due to the Earth’s atmosphere. They must also block out the central starlight, either by covering it with a small disk in the telescope or using computer software to subtract out the star’s light while retaining any glow from nearby objects such as planets.

With direct imaging, researchers have to focus their searches on young stellar systems, whose planets still retain a great deal of heat following their formation. Jupiter, an older planet, has cooled too much since it formed and now is about a billion times less bright than the sun. Directly imaged planets, still hot out of the oven, are a typically around 50,000 to 100,000 times less bright than their parent star.

With indirect techniques, astronomers are better at searching for planets that orbit close to their central star, where they cause a greater wobble or eclipsing effect. Indirect searchers, on the other hand, allow researchers to find planets much farther away from their central star, where the starlight is less overpowering. The two methods can complement one another, allowing scientists to study planets in systems at a wide variety of distances.

Above:

Brown Dwarf and Child

The first directly observed exoplanet is not quite a planet. That is, most planets we know about orbit stars. Spotted in 2004, the small red dot (the planet) is about 3 to 10 times more massive than Jupiter and is spinning around a brown dwarf, which is an object larger than a planet but without enough mass to ignite into a burning star. This odd pair is located approximately 172 light-years from Earth.

This system presents a great puzzle to astronomers, says astrophysicist Bruce Macintosh. While technically the smaller object counts as a planet, it probably didn’t form in a big dusty disk around a central star the way the planets of our solar system did, he says. Instead, it and the brown dwarf may have formed together, like a miniature version of two stars forming next to one another.

Far Out Planet

Another imaged planet orbits around a star known as IRXS J160929.1-210524, located 470 light-years away. The parent star is relatively young, only 5 million years old, and slightly smaller than the sun.

The planet is estimated to be eight times the mass of Jupiter and orbits its companion at a whopping 330 astronomical units, or more than eight times farther out than Pluto is from the sun. That such a large planet is located so far out challenges many models of planetary formation, says astronomer Ray Jayawardhana, who was on the team that imaged this object. Some models suggest the planet originally formed much closer in and was then hurled out, while others suspect the duo formed like an extremely unbalanced binary star, with one partner gobbling all the mass and the other left to starve.

Though it is probably too large and too far from its parent star to hold life, spectral data have shown that the planet has water vapor in its atmosphere.

Planetary Family

The best-studied directly imaged planets all orbit a 30-million-year-old sun-like star named HR 8799, located 129 light-years away. In 2008, data from the Keck and Gemini North telescopes on Mauna Kea in Hawaii resolved three dim companions orbiting around this star. They are estimated to be around five to seven times more massive than Jupiter and are located at roughly 25, 40, and 70 times the Earth-sun distance from their parent star.

Other telescopes have since been able to pick out the little companions and confirm their existence. Scientists have even been able to take images over several years and watch the planets as they orbit around their central star, just like the planets in our own solar system.

Family of Four

Researchers were in for a surprise when, in 2010, they found a fourth companion to the HR 8799 system. This planet was also around seven Jupiter masses and is located at 14.5 times the Earth-sun distance.

By having spectra from these worlds, researchers were able to deduce that they contain carbon monoxide and were depleted in methane, which suggests they formed in part by cannibalizing comets in their system, says Macintosh.

A Controversial Case

On the same day that the initial HR 8799 planets were announced, another team of researchers unveiled a small planet, approximately three times the mass of Jupiter, traveling through the dust disk of another young star. The star, known as Fomalhaut, is approximately 25 light-years away. Both star and planet were spotted using data from the Hubble space telescope.

But since the announcement, some astronomers have questioned the existence of the companion. During a talk at the Extreme Solar Systems conference, Jayawardhana discussed the fact that no other telescope has since been able to spot the Fomalhaut companion. The original Hubble instrument that saw it subsequently broke and will not be fixed.

Jayawardhana says the team that spotted the object reported one new data point at the conference this week. But, he says, it shows the companion behaving strangely. The initial images showed the object moving around the edge of the disk in a circular orbit, but in the newer data the planet is no longer on this trajectory. Jayawardhana does not believe
this dot is a direct image of a planet. What it is, he is not sure. Possibilities include a transient feature in the dust disk or a background star. Jayawardhana says that at the conference the team that made the initial discovery countered that the companion may have an orbit that crosses the dust disk.

Inner Planet

Another well-studied planet orbits around Beta Pictoris: a sun-like star 63 light-years away. The planet is estimated to be eight times more massive than Jupiter and orbit at only 8 astronomical units, about the distance between the sun and Saturn. Some data suggests the planet is unusually wide, and one explanation for this would be that it is surrounded by a ring of its own, perhaps making it even more like Saturn.

Going Around

Because it is much closer to its star than most other directly imaged planets, astronomers have been able to image this exoplanet at many points of its orbit. It was seen once in 2003 and again, on the other side of the star, in 2009. Researchers estimate that the planet should complete its orbit in about 15 years.

Astronomers can also see a dust-free gap around Beta Pictoris. Because the planet is in the middle of the gap, it is suspected of vacuuming up the gas and dust that exists around the young star.

Seeing in the Future

In the future, astronomers hope to take advantage of a slew of new tools that could help them directly see extrasolar planets with much higher precision. Currently being built, the Gemini Planet Imager will be deployed at the Gemini South telescope on Cerro Pachon, Chile in mid-2012. Specifically designed with planet imaging in mind, the instrument will have far superior adaptive optics and much smoother mechanics than current devices.

“It will be 10 to 100 times more sensitive than the instruments we’re using now,” said Bruce Macintosh, who is principle investigator on the project. The night sky above the Gemini South telescope can be seen in the image above.

Hot on the heels of this project are several others, including the European Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument. This telescope will be used on the Very Large Telescope (VLT) on Cerro Paranal, Chile and is also expected to see first light around mid-2012. Together, these telescopes will raise the number of directly imaged planets from a handful to many dozens.

Simulated images from the GPI depicting a hypothetical fifth companion around HR 8799 can be seen below.

Image #1: Gemini photo by Manuel Paredes, Gemini Observatory/AURA

Image #2: Marshall Perrin (STScI) and Christian Marois (HIA)

Space-Based Telescopes

Ultimately, the goal of such research would be to see an Earth-sized planet orbiting close to a star, possibly even within the habitable zone. But this day is still many years away and will require a space-based telescope mission.

NASA’s much-beleaguered James Webb Space Telescope is expected to help in direct imaging searches whenever it launches. Beyond that, other proposals involve a flotilla of probes that would block out light from a central star with a tool called a nuller. One example is the Terrestrial Planet Finder, seen here, which would require five spacecraft to split and focus light on mirrors between them. Though that project has been canceled, the technique may one day be used on a future mission.